Known belt tensioners comprise a linear piston and cylinder drive, whose piston may be driven by a compressed gas entering a working space and moves a part engaging the safety belt via a traction means attached to the piston.
A belt tensioner of this type is disclosed in the European patent publication 0 616 928 A1. The piston provided in this case is composed of a plurality of parts, between which a preferably plastically deformable damping element is arranged. On one part at the face end thereof facing the working space a bead is provided which is conically formed outward and constitutes a rigid sealing edge. A cylinder is manufactured by cutting off a suitable portion of a tube which is made by drawing and, therefore, could be of indefinite length. Since the bore diameter of the tube varies owing to wear of the tools employed for machining it, substantial variations are also liable to occur in the bore diameter from one cylinder to another. Therefore, it is not in every cylinder that a rigid sealing edge can ensure an optimum sealing effect so that in the case of a cylinder with a large bore diameter compressed gas is able to flow past the piston out of the working space, something which reduces the performance of the belt tensioner.